Superconducting rings which contain a pair of Josephson junctions and utilize quantum interference phenomenon in the measurement of magnetic fields are known. Josephson pair SQUIDS are now being constructed with thin film junctions which are as small as one micron and are separated by semiconductor or oxide insulation layers. This provides a device with high sensitivity, but also one which is subject to a high level of thermodynamically induced low frequency noise in the superconducting thin film materials. The smaller the volume of superconducting material, the greater the amount of the thermodynamic temperature fluctuations at the junction area. The effect of such fluctuations is to produce corresponding fluctuations in the operating resistance of the junction area.
The first attempts at eliminating this low frequency thermodynamically generated noise was to provide a constant DC bias current in the loop. Since the two Josephson junctions are symmetrical, the DC bias current provided a bias flux which biased the quiescent output voltage from the two junctions in opposite directions. The sensed magnetic flux was then used to amplitude modulate a modulating signal which was coupled by means of a feedback coil to the SQUID loop. The modulating signal at the two Josephson junctions, thus produced oscillatory signals which were imposed upon the steady state signal produced by the bias current and were 180 degrees out of phase.
The basic SQUID device with a constant DC bias current proved to be inadequate for many applications. In order to overcome this low frequency noise which had a l/f energy spectral density due to fluctuations at the two junctions, the circuit which was described in U.S. Pat. No. 4,389,612, issued June 21, 1983 to Michael B. Simmonds, et al, was directed to the use of a square-wave bias signal that was switched at a repetition rate that was different from the frequency of the modulating signal. According to the Simmonds patent the switching frequency of the square-wave bias signal resulted in differential noise currents due to thermo-fluctuations being reversed in phase. The fluctuation noise was not mixed or heterodyned by the switching of the bias current, but the sensed flux changes were heterodyned and appeared as side bands around the switching frequency and its harmonics. A modulating AC signal was also employed to demodulate the resulting output signal to recover the original input signal.
while the circuit of the Simmonds, et al patent was an improvement in eliminating low frequency noise in multijunction SQUIDS the employment of square-wave bias switching at one frequency and modulation at another frequency in itself generated additional and undesired noise components due to cross modulation frequency products from the two different frequencies employed. The present invention is directed to a circuit in which low frequency thermodynamic fluctuation noise is avoided without the production of this cross-modulation noise. In the present invention this is accomplished by use of a signal frequency both for switching the square-wave bias signal and for modulating the input, wherein a phase shift of approximately 90.degree. is maintained between the switching frequency and the modulation signal.